Semiconductor magneto-optical material

ABSTRACT

A semiconductor magneto-optical material includes a semiconductor dispersed with fine magnetic material particles and is characterized by exibiting magneto-optical optical effect at ordinary room temperature.

This application is a division of application Ser. No. 09/007,515 filedJan. 15, 1998, now U.S. Pat. No. 6,132,524.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a semiconductor magneto-optical material whosemagneto-optical effect can be utilized to enable optical communication,signal processing and data recording by laser beam or the like.

2. Description of the Prior Art

Recent advances in next-generation optical communication technology andnext-generation large-capacity data storage technology, together withever expanding networks offering commercial connection services and thelike via the Internet, are combining to set the stage for a full-blownmultimedia age that is rapidly approaching.

The core technologies required for next-generation opticalcommunications are:

1. Ultrahigh speed/ultra-long distance transmission technology

2. Optical coherent communication technology

3. Optical signal processing technology

4. Optical component and integrated circuit technology.

Of recent technological breakthroughs, one of the most significant inconnection with items 1-4 is the erblum-doped fiber amplifier (EDFA).Owing to its larger repercussion effect, the EDFA has dramaticallyincreased transmission distance.

A laser diode (LD) module is used as the excitation light source and thesignal light source of the EDFA. When a laser module is used, however,light reflected by, for example, the end face of the optical fiberconnected with the laser module and the connection points betweenoptical fibers reenters the LD. The operation characteristics aretherefore markedly degraded by the occurrence of retrogressive lightnoise, output fluctuation, and other factors. The practice is thereforeto block the retrogressive light reflected toward the LD by use of anoptical isolator so as to overcome the operational instability of the LDmodule owing to reflected retrogressive light.

An optical isolator is an optical component (optical nonreciprocalcircuit) using a magneto-optical material exhibiting magneto-opticalFaraday effect. In the field of optical communication, optical isolatorshave been developed for various wavelengths, including the 0.8 μm band(the wavelength of the most inexpensive GaAs semiconductor laser), the1.3 μm-1.5 μm band (the band of lowest optical fiber transmission loss)and the 0.98 μm band (used for high-efficiency EDFA excitation).

Except in the 0.98 μm band, the typical magneto-optical material used isbismuth-substituted garnet.

Since the optical absorption of bismuth-substituted garnet is large inthe 0.98 μm band, however, another magneto-optical material has beensought. This led to the recent development of a practical bulk isolatorusing a magnetic semiconductor based on cadmium telluride, a II-VI groupsemiconductor.

On the other hand, the optical isolator continues to account for a majorportion of optical amplifier size and cost. In view of plans to connectindividual homes with optical communication networks for introduction ofbidirectional interactive services, multimedia communication servicesand the like, a strong need is felt for a smaller, low-cost opticalisolator and for an optical waveguide-type optical isolator in the formof a thin film on the surface of a substrate. It will be immeasurable ifthis need should be met.

When a magnetic semiconductor based on cadmium telluride is used as themagneto-optical material of the optical isolator, the magneto-opticalmaterial must have a thickness of around 1,400 μm in order to secure a45-degree rotation angle of the polarization surface. This makes itdifficult to achieve small size and low cost.

An attempt to fabricate an optical isolator as an optical waveguide-typeoptical isolator encounters considerable difficulty in realizing theoptical waveguide since bismuth-substituted garnet and cadmium tellurideare poorly compatible with the GaAs semiconductor of the substrate.

An optical switching element utilizing the magneto-optical effect isalso desired, not only for use in optical isolators but also forrealizing optical integrated circuits, optical computers and the like. Amagneto-optical material that can overcome the foregoing problems istherefore also sought for this purpose.

This invention was accomplished in light of the foregoing circumstancesand has as its object to provide a semiconductor magneto-opticalmaterial that exhibits pronounced magneto-optical effect in a desiredwavelength region and can be formed as a thin film.

SUMMARY OF THE INVENTION

To achieve this object, the invention provides a semiconductormagneto-optical material comprising a semiconductor dispersed with finemagnetic material particles, which is characterized by exhibitingmagneto-optical effect at ordinary room temperature.

Since the energy gap of the semiconductor constituting the matrix can befreely changed, the semiconductor magneto-optical material can beadapted to any desired wavelength region.

The magnitude of the magneto-optical effect (Faraday effect) can berepresented in terms of the thickness of the medium that rotates thepolarization surface of the light by 45 degrees. The thickness of thesemiconductor magneto-optical material of this invention required torotate the polarization surface of light of 0.98 μm wavelength by 45degrees is 300 μm, about one-fifth that required in the case of thecurrently used magnetic semiconductor based on cadmium telluride. Sincethe material can therefore exhibit the required properties even as athin film, it is capable of reducing size and lowering cost.

The above and other features of the present invention will becomeapparent from the following description made with reference to thedrawings.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is a sectional view showing an example of the structure of aspecimen of the semiconductor magneto-optical material according to theinvention.

FIG. 2 is a schematic view of a measurement system for measuring themagneto-optical effect of the semiconductor magneto-optical materialaccording to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The inventors discovered that dispersion of fine magnetic materialparticles in a semiconductor results in a semiconductor magneto-opticalmaterial exhibiting a large magneto-optical effect in freely selectablewavelength region at room temperature. This invention was accomplishedon the basis of this discovery.

The semiconductor of the matrix can be any IV group semiconductor suchas Si, III-V group semiconductor, of which GaAs is typical, or the likeenabling dispersion of fine magnetic material particles. Since theenergy gap of the semiconductor can be changed as desired, thesemiconductor magneto-optical material can be adapted to any wavelengthregion.

Fine magnetic material particles usable in the invention include thoseof such magnetic elements as Fe, Co and Ni and those of such magneticcompounds as MnAs, MnGa, MnSb and MnAl. Magnetic properties of the finemagnetic material particles such as magnetic transition temperature andsaturation magnetic field can be controlled by mixing two or more typesof particles and/or by appropriately adding thereto one or more of theelements Al, Si, Ti, V, Cr, Cu Zn, Zr, Nb, Mo, To, Ru, Rh, Pd, Ag, Cd,In, Sn, Te, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb and Bi.

Methods usable for dispersing the fine magnetic material particles inthe semiconductor include the molecular beam epitaxy method, suchsimultaneous vapor deposition methods as the vapor phase growth method,sputtering method and laser ablation method, and methods combining anyof these with heat, laser beam or electron beam treatment. Any methodcapable of forming fine magnetic material particles in the semiconductorsuffices.

An embodiment of the invention will now be explained in detail withreference to the drawings.

This embodiment relates to fabrication of a MnAs:GaAs semiconductormagneto-optical material specimen and measurement of the magneto-opticaleffect exhibited by the specimen.

FIG. 1 is a sectional view showing an example of the structure of theMnAs:GaAs semiconductor magneto-optical material specimen. The thin film1 in FIG. 1 is composed of a semiconductor magneto-optical materialaccording to the invention and in this embodiment is a film ofMnAs:GaAs. Specifically, the thin film 1 was formed to a thickness ofabout 100 nm by molecular beam epitaxy conducted at 220° C. to uniformlydisperse fine particles of MnAs, a magnetic material, in the GaAs, asemiconductor material, on a GaAs semiconductor substrate 2. The resultwas annealed at about 580° C. for about 10 min. to obtain a specimen 10in FIG. 2. The Mn concentration of the thin film 1 was about 4% relativeto the Ga concentration. The particle diameter of the MnAs in the thinfilm was approximately 10 nm.

GaAs was selected as the material for both the matrix semiconductor ofthe semiconductor magneto-optical material and the substrate thereof.This was to show that the magneto-optical material can be easilyintegrated with GaAs semiconductor, which is the semiconductor currentlymost often used for laser fabrication in optical communicationtechnologies.

Measurement of the magneto-optical effect of the thin film 1 wasperformed using the GaAs semiconductor substrate 2 and the thin film 1as the specimen 10, with a laser beam (indicated by the arrow in FIG. 1)directed perpendicular to the specimen 10.

FIG. 2 is a schematic view of a measurement system for measuring themagneto-optical effect of the semiconductor magneto-optical material(thin film 1) according to the invention. The measurement systemcomprises a pair of samarium-cobalt magnets 7A, 7B, Glan-Thompson prisms6A, 6B disposed on opposite sides of the samarium-cobalt magnets 7A, 7B,and a power meter 8 disposed as the final stage. The specimen 10 isdisposed in the magnetic field between the samarium-cobalt magnets 7A,7B. A semiconductor laser 3 outputting a laser beam of 0.98 μmwavelength is used as the light source. During measurement, the laserbeam from the semiconductor laser 3 is directed into the Glan-Thompsonprism 6A constituting the first stage of the measurement system throughan optical isolator 4 and a lens 5. The measurement was effected at roomtemperature.

In the measurement system of the foregoing configuration, the laser beamexiting the Glan-Thompson prism 6A impinges perpendicularly on thespecimen 10 and has its polarization surface rotated by a prescribedangle owing to the Faraday effect of the thin film 1. The rotation angleis measured by the Glan-Thompson prism 6B and the power meter 8. Themeasurement system is configured with application of the inventionsemiconductor magneto-optical material to optical isolators in mindbecause a particularly urgent need is currently felt for the developmentof high-performance optical isolators for high-efficiency EDFAexcitation.

The results of the performance evaluation by the measurement systemdescribed above are shown in the following table.

Thickness required for Quenching 45-degree rotation ratio Invention 300μm 38 dB (MnAs:GaAs) (at 0.98 μm) CdMnHgTe 1,400 μm   30 dB (at 0.98 μm)Bismuth-substituted 250 μm >30 dB  garnet (at 1.3 μm) 

The magnitude of the magneto-optical effect (Faraday effect) isindicated in terms of the thickness of the medium (material) required torotate the polarization surface of the light 45 degrees. (In the actualoptical isolator the element thickness is adjusted to rotate thepolarization surface by 45 degrees.)

The Faraday rotation angle θF indicating the Faraday effect is generallyrepresented by the equation:

θF [degree]=V [degree/Oe/μm]×5000 [Oe]×Material thickness [μm],

where V is Verdet's constant, a coefficient representing Faradayrotation magnitude, and 5000 Oe is the strength of the magnetic fieldapplied by the magnets 7A, 7B.

It can be seen from this equation that the thickness of the materialrequired to rotate the polarization surface of the light by 45 degreesis inversely proportional to the value of Verdet's constant V. Thethickness of the semiconductor magneto-optical material can therefore bemade thinner in proportion as Verdet's constant V is larger.

As shown in the table above, the thickness of the material required torotate the polarization surface of the light of 0.98 μm wavelength by 45degrees is 300 μm for the MnAs:GaAs of the invention in comparison with1,400 μm (1.4 mm) for CdMnHgTe, the magnetic semiconductor based oncadmium telluride currently in practical use. In other words, theinvention achieves about 4.6 times the performance of the magneticsemiconductor based on cadmium telluride. As further shown by way ofcomparison, this performance is on substantially the same order as thatof the practically applied bismuth-substituted garnet at a wavelength of1.3 μm.

Since the semiconductor magneto-optical material of the invention has avery large Verdet's constant, it reduces the required material thicknessand, as such, can reduce the size of magneto-optical elements and beused in the form of thin film. Since the material is semiconductorbased, moreover, it has good compatibility with a semiconductorsubstrate and can therefore be easily formed on a semiconductorsubstrate as a thin film. It therefore also enables ready realization ofan optical waveguide-type optical isolator.

A condition for the application of the material to an actual element. isthat its quenching ratio be not less than 30 dB when the crossed-nicholcondition defined by the equation below is satisfied. Since theperformance evaluation showed it to have a quenching ratio of 38 dB, thesemiconductor magneto-optical material according to the inventioncomposition system clearly achieves a level of performance on this pointadequate for practical application.

Quenching ratio=(Crossed-nichol optical transmittance)/(Parallel nicholoptical transmittance)

Moreover, it was ascertained that the rotation angle magnitude andmagnetic field sensitivity of the material can be controlled by varyingthe size of the fine magnetic material particles and that the materialdoes not experience change with aging. The semiconductor magneto-opticalmaterial according to the invention is thus free of any problemhindering its application as a magneto-optical material.

Since the MnAs:GaAs material of this embodiment exhibits largemagneto-optical effect even at room temperature, it achieves superiorperformance even as a thin film and can therefore contribute to size andcost reduction.

The foregoing explanation focuses on the Faraday effect of thesemiconductor magneto-optical material according to the invention sincethis is the most important property from the point of application to anoptical isolator. However, the material also exhibits othermagneto-optical effects, such as Cotton-Mouton effect and magnetic Kerreffect, of similar magnitude.

Although GaAs was used as the semiconductor for the matrix in theembodiment described, any semiconductor can be used insofar as itenables dispersion of the fine magnetic material particles, including,for example, Si and other IV group semiconductors and III-V groupsemiconductors other than GaAs. Since the energy gap of a semiconductorcan be changed as desired, the semiconductor magneto-optical materialcan be adapted to any wavelength region.

Although particles of MnAs, a magnetic compound, were used as the finemagnetic material particles, other fine magnetic material particles arealso usable, including particles of magnetic elements such as Fe, Co andNi and particles of magnetic compounds such as MnGa, MnSb and MnAl.Further, magnetic properties of the fine magnetic material particlessuch as magnetic transition temperature and saturation magnetic fieldcan be controlled by mixing two or more types of particles and/or byappropriately adding thereto one or more of the elements Al, Si, Ti, V,Cr, Cu Zn, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Te, Hf, Ta, W,Re, Os, Ir, Pt, Au, Hg, Tl, Pb and Bi.

Methods usable for dispersing the fine magnetic material particles inthe semiconductor include not only the molecular beam epitaxy methodemployed in the embodiment but also such simultaneous vapor depositionmethods as the vapor phase growth method, sputtering method and laserablation method, and methods combining any of these with heat, laserbeam or electron beam treatment. Any method capable of forming finemagnetic material particles in the semiconductor suffices.

The semiconductor magneto-optical material according to the inventioncan be readily used to fabricate more compact, less expensive, higherperformance elements by use of conventional semiconductor technologies.Since the material is semiconductor based, moreover, it enablesfabrication of an integrated device including both a semiconductor laserand a waveguide-type optical isolator, realization of thin film opticalswitches in semiconductor optical integrated circuits, and size and costreduction of such devices and elements.

As will be understood from the foregoing explanation, the inventionmakes it possible to fabricate a semiconductor magneto-optical materialexhibiting large magneto-optical effect by dispersing fine magneticmaterial particles in a semiconductor.

In accordance with the invention, a semiconductor magneto-opticalmaterial exhibiting large magneto-optical effect in any desiredwavelength region even at room temperature can be obtained byappropriate selection of the semiconductor of the matrix. The materialsaccording to the invention can therefore be used in place of all of thevarious magneto-optical materials corresponding to different wavelengthregions employed in currently used optical isolators. Since theinvention materials exhibit large magneto-optical effect, moreover, theycan fulfill their functions even in the form of thin films. Theinvention therefore makes a significant contribution to size and costreduction.

What is claimed is:
 1. A method of switching the polarization of lightpenetrating a semiconductor magneto-optical material, comprising:irradiating the semiconductor magneto-optical material with a light inthe presence of a magnetic field; wherein said semiconductormagneto-optical material comprises a semiconductor having dispersedtherein fine magnetic compound particles selected from the groupconsisting of MnAs, MnGa, MnSb and MnAl.
 2. The method of claim 1,wherein said semiconductor is a group IV semiconductor or a group III-Vsemiconductor.